TOPIC 2: PHOTOSYNTHESIS

Photosynthesis is the way in which plants make their own food (glucose) using light energy from the sun.

Energy carriers:

Photosynthesis also uses ADP and ATP as outlined above in respiration. Note: Remember link the P in NADP+ and NADPH to Photosynthesis

Photosynthesis is divided into two stages 1. The light stage 2. The dark stage

Photosynthesis will use the large amount of energy stored in ATP and in the high energy electrons to make glucose (anabolic reaction)

Stage 1: Light Stage/Light Dependent Stage (requires light) • Location: The chloroplast (membranes containing chlorophyll)

• Occurs due to the movement of electrons

• Reactions are not controlled by enzymes

• Events in the light stage include

1. Light is absorbed

2. Light energy is transferred to electrons

3. High energy electrons enter either

• The cyclic pathway (Electron pathway 1) • The non-cyclic pathway (Electron pathway 2) Pathway 1: Cyclic pathway

• The cyclic pathway is made up of electron acceptors

• The high energy electrons leave the electron acceptor and move along the electron acceptor pathway

• As they move the high energy electrons lose their energy which is picked up by ADP to form ATP

• The electrons return to the reaction centre chlorophyll having lost their energy Pathway 2: Non-cyclic pathway • Two high-energy electrons pass from the electron acceptor into the pathway • These high-energy electrons lose some energy which is used to form ATP • The two high-energy electrons join with NADP+ to form NADP

• Two water molecules (H2O) are split using light energy

Products of water splitting

• Four hydrogen protons (4H+) ● Some are stored in a pool of protons ● One hydrogen proton H+ is attracted to the NADP- to form NADPH

• Four electrons (4e-) ● Two of these electrons are returned to the chlorophyll reaction centre • Oxygen ● The oxygen produced diffuses out of the chloroplast Pathway 2: Non-cyclic pathway Stage 2: The Dark Stage/Light Independent Stage (does not require light)

• Location: Chloroplasts (Stroma)

• Controlled by enzymes

● Dark stage is affected by temperature ● Enzymes can become denatured above or below the optimum temperature (20 – 30°C) • Carbon dioxide (provides the carbon to make glucose) diffuses into the chloroplast

• NADPH and ATP enter the stroma in the chloroplast

• NADPH breaks down releasing hydrogen ions H+ and electrons

• ATP releases its energy forming ADP

• This energy is used to combine carbon dioxide, hydrogen protons (H+) and electrons to form glucose

• ADP and NADP+ are reused in the light stage 2006 Question 11(a)(b) 33 marks (a)

(i) What is the primary role of chlorophyll in

photosynthesis? Traps light energy from the sun (3)

(ii) Write an equation to summarise photosynthesis

(b) The second stage of photosynthesis is called the dark stage or light independent stage.

(i) Why is the dark stage given the alternative name of the light-independent stage?

It does not use/need light (3)

(ii) Name a gas that is essential for the dark stage Carbon dioxide (3)

(iii) Two products of the light stage are vital for the dark stage. Name each of them.

NADPH and ATP (2x3)

(iv) State the precise role in the dark stage of each of the substances that you named in (iii) NADPH: Supplies hydrogen protons and high-energy electrons to the dark stage (3)

ATP: Supplies energy to make glucose (3)

(v) To what group of biomolecules do the main products of the dark stage belong? Carbohydrates/monosaccharide’s (6)